The protein CCL7 (which regulates migration of white blood cells in inflammation) consists of a 3-stranded β-sheet packed against an α-helix.

The side chain of residue X forms ionic interactions with the side chains of both Lys-58 and Lys-65 on the surface of the α-helix. This interaction stabilises the folded structure of CCL7 at neutral pH (but not at pH 3 or pH 11).

Examine the structure of CCL7 in the PDB 3D View or by downloading the PDB file 1BO0 into PyMOL.

What is the most likely identity of residue X?